Because of their components (optical fibers, amplifiers, etc.), optical transmission lines induce power losses that degrade transmission performance and can in some circumstances lead to processing errors and even to loss of data. These optical power losses are usually related to aging of the components and/or to maintenance or repair work.
To track changing power losses it has been proposed to measure periodically the optical power of the signals at the exit end of optical fibers. That type of measurement cannot distinguish a loss induced by an optical fiber from a loss induced by an optical amplifier that feeds optical signals into the fiber.
U.S. Pat. No. 6,366,381 describes a method of remotely measuring the power at the output of a repeater which is part of a long optical link and of transmitting the measurement result to a first terminal at a first end of the link. A photodetector at the output of the repeater supplies an electrical signal representing the output power. The gain of a fiber amplifier included in the repeater is modulated to transmit the measurement result to the first terminal. The amplifier amplifies a supervisory optical signal that is sent by a second terminal located at the second end of the link. The amplitude modulation index is proportional to the measured output power. It then suffices to measure the modulation index of the supervisory signal, when it reaches the first terminal, to determine the output power of the repeater.
It has also been proposed to measure the optical power at the entry and exit ends of the fiber and to send the measurement results to a comparator module using supervisory binary signals. This necessitates a dedicated supervisory channel, which may not be available, and requires processing time that may not be suitable for dynamically adjusting the optical power.